Hard disk drives are used in almost all computer system operations. In fact, most computing systems are not operational without some type of hard disk drive to store the most basic computing information such as the boot operation, the operating system, the applications, and the like. In general, the hard disk drive is a device which may or may not be removable, but without which the computing system will generally not operate.
The basic hard disk drive model includes a storage disk or hard disk that spins at a designed rotational speed. An actuator arm with a suspended slider is utilized to reach out over the disk. The slider includes a head assembly that has a magnetic read/write transducer or head for reading/writing information to or from a location on the disk. The complete-head assembly, e.g., the suspension, slider, and head, is called a head gimbal assembly (HGA).
In operation, the hard disk is rotated at a set speed via a spindle motor assembly having a central drive hub. There are tracks at known intervals across the disk. When a request for a read of a specific portion or track is received, the hard disk aligns the head, via the arm, over the specific track location and the head reads the information from the disk. In the same manner, when a request for a write of a specific portion or track is received, the hard disk aligns the head, via the arm, over the specific track location and the head writes the information to the disk.
Over the years, the disk and the head have undergone great reductions in their size. Much of the refinement has been driven by consumer demand for smaller and more portable hard drives such as those used in personal digital assistants (PDAs), Moving Picture Experts Group audio layer 3 (MP3) players, and the like. For example, the original hard disk drive had a disk diameter of 24 inches. Modern hard disk drives are much smaller and include disk diameters of less than 2.5 inches. Advances in magnetic recording are also primary reasons for the reduction in size.
However, the small drives have small components with very narrow tolerances. Disk drive sliders are designed to fly in very close proximity to the disk surface. For instance, in some systems the slider may be designed to fly only three to five nanometers above the disk surface. In a system with such close tolerances, components can be subject to van der Waals, Meniscus, electrostatic, spindle motor charge up, and contact potential forces. These forces are due to a variety of causes, such as: the molecular attraction of components in very close proximity; adhesive friction caused by contact between the slider and the lubricant on the disk; the build up of electrical potential between the disk and the slider caused by the rotating disk surface (tribo-charging); the build up of electrical potential in motor bearings (tribo-charging); potential difference that exists between two different metals (different Fermi levels of slider and disk material); and impacts between the slider and disk surface. These forces alone, and in combination, create bouncing vibrations of the slider.
Bouncing vibrations of the slider are undesirable because they can cause media damage. Bouncing vibrations also cause variations in the magnetic spacing between the head element and media that are likely to cause data errors—both hard errors during writing and soft errors during reading. The bouncing vibration that causes the most concern occurs in the second pitch mode of the slider. This ‘Pitch 2’ mode vibration is typically around 250 kHz (kilo Hertz) and is incoherent.
One approach to reducing these bouncing vibration problems is to use rougher media or a padded slider. However, due to possible wear on these features, the actual contact area changes over time, often leading to more bounce vibrations. Increased damping of the slider through improved air bearing design is another approach. This provides some help in high disk RPM (revolutions per minute) applications, but is of little help at low disk RPM applications because the dampening force is far too weak to overcome the forces that cause the bounce vibrations. For instance, in low disk RPM applications, the Pitch 2 mode damping coefficient is usually less than 3% of the critical damping coefficient.
Another approach to reducing vibrations is electrostatic fly height control. This sort of active servo control of the slider can work well, but is difficult to implement in a functioning hard disk drive because it is complex and requires very precise measurements regarding fly height.